This invention relates to the addressing of matrix array type ferro-electric liquid crystal display devices.
Hitherto dynamic scattering mode liquid crystal display devices have been operated using a d.c. drive or an a.c. one, whereas field effect mode liquid crystal devices have generally been operated using an a.c. drive in order to avoid performance impairment problems associated with electrolytic degradation of the liquid crystal layer. Such devices have employed liquid crystals that do not exhibit ferro-electricity, and the material interacts with an applied electric field by way of an induced dipole. As a result, they are not sensitive to the polarity of the applied field, but respond to the applied RMS voltage averaged over approximately one response time at that voltage. There may also be frequency dependence as in the case of so-called two-frequency materials, but this only affects the type of response produced by the applied field.
In contrast to this, a ferro-electric liquid crystal exhibits a permanent electric dipole, and it is this permanent dipole which will interact with an applied electric field. Ferro-electric liquid crystals are of interest in display applications because they are expected to show a greater coupling with an applied field than that typical of a liquid crystal that relies on coupling with an induced dipole, and hence ferro-electric liquid crystals are expected to show a faster response. A ferro-electric liquid crystal display mode is described for instance by N. A. Clark et al. in a paper entitled `Ferro-Electric Liquid Crystal Electro-Optics Using the Surface Stabilized Structure` appearing in Mol. Cryst. Liq. Cryst. 1983 volume 94 pages 213 to 234. Two properties of ferro-electrics set the problems of matrix addressing such devices apart from the addressing of non-ferro-electric devices. First they are polarity sensitive, and second their response times exhibit a relatively weak dependence upon the applied voltage. The response time of a ferro-electric is typically proportional to the inverse square of applied voltage, or even worse, proportional to the inverse single power of the voltage; whereas a non-ferro-electric smectic A, which in certain other respects is a comparable device exhibiting long term storage capability, exhibits a response time that is typically proportional to the inverse fifth power of the voltage.
The use of ferro-electric displays is therefore restricted by difficulties in addressing the display. If such a display is addressed via a conventional X-Y matrix, then interference analogous to cross-talk prevents the minimum response time from being achieved. Application of a signal to a row or column of a display can cause changes in the state of pixels other than the particular one being addressed.